Now, in the market of optical recording medium there have been distributed rewritable optical recording media and so-called write-once optical recording media which are not rewritable. A rewritable optical recording medium, as meant by the word “rewritable,” allows data to be written therein again and again, so that it is possible for the same optical recording medium to be used repeatedly in recording only required data. On the other hand, a write-once optical recording medium is not rewritable and thus characterized by a feature that “data will never be altered,” thereby making itself useful in data distribution, storage, or backup.
Conventionally, as a recording structure of a write-once optical recording medium, it has been suggested that organic dyes is applied to a substrate. However, such a recording structure has been found insufficient in its recording sensitivity when performing a high speed recording. Furthermore, if the wavelength of a laser beam is made short in order to increase a recording density, there will be a problem that it is difficult to synthesize dyes which can be used with laser beams having wavelengths equal to or shorter than that of a blue light.
Although there have been several suggestions (for example, Japanese Patent Publication No. 1992-838, etc.) that recording layer can be formed by an inorganic material, none of conventional structures is suitable for high density or high speed recording, their storage reliability of recorded state is inadequate, and their playback durability is insufficient.
For this reason, among various high density optical recording media using laser beams having wavelengths equal to or shorter than that of a blue light, what has been suggested as an optical recording medium capable of “high speed recording” is only a rewritable (RW) optical recording medium formed by using a phase-change material.
Recently, in the field of optical recording media for use as a multimedia-compatible medium, there has been a demand for a higher density and higher speed recording. Similarly, the same demand is existing in the field of write-once optical recording media.
With regard to a rewritable optical recording medium, it is necessary to strictly control several time-related factors such as cooling speed, with the recording strategy becoming more complex because of high speed and high density recording. On the other hand, a write-once optical recording medium has a recording strategy which was not as complex as a rewritable optical recording medium. However, in view of a further higher density and further high speed recording, what has been clearly understood is that it is impossible to obtain sufficient characteristics by a recording strategy using a conventional write-once optical recording medium. Here, the recording strategy means a power control pattern of a recording laser beam. Generally speaking, a recording laser beam (especially when a recording is performed on an optical recording medium using a phase-change material) is not continuously irradiated corresponding to the length of recording mark. In contrast, as described in Japanese Patent Laid-Open Publication No. 1997-7176, for the purpose of controlling the shape of recording mark, a general practice is to irradiate a laser beam formed by a pulse train consisting of a plurality of pulses, with the width of each pulse in the pulse train strictly controlled. At this time, detailed arrangement of pulse dividing is usually designated to as recording strategy.